U.S. patent number 11,214,856 [Application Number 16/489,649] was granted by the patent office on 2022-01-04 for ferritic stainless steel sheet, hot coil, and automobile exhaust flange member.
This patent grant is currently assigned to NIPPON STEEL CORPORATION. The grantee listed for this patent is NIPPON STEEL CORPORATION. Invention is credited to Akihiro Nishimura, Shinichi Tamura, Shinichi Teraoka.
United States Patent |
11,214,856 |
Teraoka , et al. |
January 4, 2022 |
Ferritic stainless steel sheet, hot coil, and automobile exhaust
flange member
Abstract
A ferritic stainless steel plate having a sheet thickness t of
5.0 to 12.0 mm, including a chemical composition consisting of, in
mass percent, C: 0.001 to 0.010%, Si: 0.01 to 1.0%, Mn: 0.01 to
1.0%, P: 0.04% or less, S: 0.010% or less, Cr: 10.0 to 20.0%, Ni:
0.01 to 1.0%, Ti: 0.10 to 0.30%, V: 0.01 to 0.40%, Al: 0.005 to
0.3%, N: 0.001 to 0.02%, and as necessary, one or more of B, Mo,
Cu, Mg, Sn, Sb, Zr, Ta, Nb, Hf, W, Co, Ca, REM, and Ga, with the
balance being Fe and unavoidable impurities, wherein in a steel
micro-structure, on a cross section parallel to a rolling
direction, an area ratio of structures each satisfying: major grain
diameter/minor grain diameter being 5.0 or more is 90% or more, and
an average minor grain diameter of the structures is 100 .mu.m or
less.
Inventors: |
Teraoka; Shinichi (Tokyo,
JP), Tamura; Shinichi (Tokyo, JP),
Nishimura; Akihiro (Tokyo, JP) |
Applicant: |
Name |
City |
State |
Country |
Type |
NIPPON STEEL CORPORATION |
Tokyo |
N/A |
JP |
|
|
Assignee: |
NIPPON STEEL CORPORATION
(Tokyo, JP)
|
Family
ID: |
61195734 |
Appl.
No.: |
16/489,649 |
Filed: |
February 28, 2017 |
PCT
Filed: |
February 28, 2017 |
PCT No.: |
PCT/JP2017/007965 |
371(c)(1),(2),(4) Date: |
August 28, 2019 |
PCT
Pub. No.: |
WO2018/158853 |
PCT
Pub. Date: |
September 07, 2018 |
Prior Publication Data
|
|
|
|
Document
Identifier |
Publication Date |
|
US 20200002793 A1 |
Jan 2, 2020 |
|
Current U.S.
Class: |
1/1 |
Current CPC
Class: |
C22C
38/005 (20130101); C21D 6/004 (20130101); C22C
38/001 (20130101); C22C 38/06 (20130101); C22C
38/50 (20130101); C21D 9/46 (20130101); C22C
38/60 (20130101); C22C 38/04 (20130101); C22C
38/42 (20130101); C22C 38/52 (20130101); C22C
38/008 (20130101); C21D 8/0205 (20130101); C22C
38/46 (20130101); C22C 38/54 (20130101); C22C
38/02 (20130101); C22C 38/44 (20130101); C22C
38/00 (20130101); C22C 38/004 (20130101); C21D
8/0226 (20130101); C21D 8/02 (20130101); C22C
38/002 (20130101); C22C 38/48 (20130101); F16L
23/032 (20130101); C21D 2211/005 (20130101) |
Current International
Class: |
C21D
6/00 (20060101); F16L 23/032 (20060101); C22C
38/52 (20060101); C22C 38/54 (20060101); C22C
38/50 (20060101); C22C 38/48 (20060101); C22C
38/46 (20060101); C22C 38/44 (20060101); C22C
38/42 (20060101); C22C 38/06 (20060101); C22C
38/04 (20060101); C22C 38/02 (20060101); C21D
9/46 (20060101); C22C 38/00 (20060101); C21D
8/02 (20060101); C22C 38/60 (20060101) |
Field of
Search: |
;420/36 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
|
|
|
|
|
|
|
105121685 |
|
Dec 2015 |
|
CN |
|
60-228616 |
|
Nov 1985 |
|
JP |
|
8-199237 |
|
Aug 1996 |
|
JP |
|
2000-169943 |
|
Jun 2000 |
|
JP |
|
2012-140687 |
|
Jul 2012 |
|
JP |
|
2012-140688 |
|
Jul 2012 |
|
JP |
|
2015187290 |
|
Oct 2015 |
|
JP |
|
2015-190025 |
|
Nov 2015 |
|
JP |
|
WO 2015/147211 |
|
Oct 2015 |
|
WO |
|
Other References
International Search Report for PCT/JP2017/007965 dated May 30,
2017. cited by applicant .
Notice of Reasons for Refusal for JP 2017-536601 dated Sep. 5,
2017. cited by applicant .
Written Opinion of the International Searching Authority for
PCT/JP2017/007965 (PCT/ISA/237) dated May 30, 2017. cited by
applicant .
International Preliminary Report on Patentability and English
translation of the Written Opinion of the International Searching
Authority (Forms PCT/IB/326, PCT/IB/373, and PCT/ISA/237) for
International Application No. PCT/JP2017/007965, dated Sep. 12,
2019. cited by applicant.
|
Primary Examiner: Sheikh; Humera N.
Assistant Examiner: Christy; Katherine A
Attorney, Agent or Firm: Birch, Stewart, Kolasch &
Birch, LLP.
Claims
The invention claimed is:
1. A ferritic stainless steel sheet having a sheet thickness t of
5.0 to 12.0 mm, comprising a chemical composition consisting of, in
mass percent: C: 0.001 to 0.010%; Si: 0.01 to 1.0%; Mn: 0.01 to
1.0%; P: 0.04% or less; S: 0.010% or less; Cr: 10.0 to 20.0%; Ni:
0.01 to 1.0%; Ti: 0.10 to 0.30%; V: 0.01 to 0.40%; Al: 0.005 to
0.3%; N: 0.001 to 0.02%; B: 0 to 0.0030%; Mo: 0 to 2.0%; Cu: 0 to
0.3%; Mg: 0 to 0.0030%; Sn: 0 to 0.1%; Sb: 0 to 0.1%; Zr: 0 to
0.1%; Ta: 0 to 0.1%; Nb: 0 to 0.1%; Hf: 0 to 0.1%; W: 0 to 0.1%;
Co: 0 to 0.2%; Ca: 0 to 0.0030%; REM: 0 to 0.05%; Ga: 0 to 0.1%;
and Bi: 0 to 0.1%, with the balance being Fe and unavoidable
impurities, wherein in a steel micro-structure, on a cross section
parallel to a rolling direction, an area ratio of structures each
satisfying: major grain diameter/minor grain diameter being 5.0 or
more is 90% or more, wherein the structures are a zone of at least
1 mm.sup.2 at a position of 0.25 of the sheet thickness and 0.50 of
the sheet thickness, and an average minor grain diameter of the
structures is 100 .mu.m or less, wherein the structures are
measured on a line parallel to a sheet thickness direction within a
range of 0.25 of the sheet thickness to 0.75 of the sheet
thickness.
2. A hot coil made of the ferritic stainless steel sheet according
to claim 1.
3. An automobile exhaust flange member made of the ferritic
stainless steel sheet according to claim 1.
4. An automobile exhaust flange member made using the ferritic
stainless hot coil according to claim 2.
5. A ferritic stainless steel sheet having a sheet thickness t of
5.0 to 12.0 mm, comprising a chemical composition comprising, in
mass percent: C: 0.001 to 0.010%; Si: 0.01 to 1.0%; Mn: 0.01 to
1.0%; P: 0.04% or less; S: 0.010% or less; Cr: 10.0 to 20.0%; Ni:
0.01 to 1.0%; Ti: 0.10 to 0.30%; V: 0.01 to 0.40%; Al: 0.005 to
0.3%; N: 0.001 to 0.02%; B: 0 to 0.0030%; Mo: 0 to 2.0%; Cu: 0 to
0.3%; Mg: 0 to 0.0030%; Sn: 0 to 0.1%; Sb: 0 to 0.1%; Zr: 0 to
0.1%; Ta: 0 to 0.1%; Nb: 0 to 0.1%; Hf: 0 to 0.1%; W: 0 to 0.1%;
Co: 0 to 0.2%; Ca: 0 to 0.0030%; REM: 0 to 0.05%; Ga: 0 to 0.1%;
and Bi: 0 to 0.1%, with the balance comprising Fe and unavoidable
impurities, wherein in a steel micro-structure, on a cross section
parallel to a rolling direction, an area ratio of structures each
satisfying: major grain diameter/minor grain diameter being 5.0 or
more is 90% or more, wherein the structures are a zone of at least
1 mm.sup.2 at a position of 0.25 of the sheet thickness and 0.50 of
the sheet thickness, and an average minor grain diameter of the
structures is 100 .mu.m or less, wherein the structures are
measured on a line parallel to a sheet thickness direction within a
range of 0.25 of the sheet thickness to 0.75 of the sheet
thickness.
6. A hot coil made of the ferritic stainless steel sheet according
to claim 5.
7. An automobile exhaust flange member made of the ferritic
stainless steel sheet according to claim 5.
8. An automobile exhaust flange member made using the ferritic
stainless hot coil according to claim 6.
Description
TECHNICAL FIELD
The present invention relates to a ferritic stainless steel sheet,
a hot coil, and an automobile exhaust flange member.
BACKGROUND ART
An exhaust gas passage of an automobile is made up of various
components including an exhaust manifold, an exhaust gas
recirculation (EGR), a muffler, a catalyst, a Diesel particulate
filter (DPF), a urea selective catalytic reduction (SCR), a
flexible tube, a center pipe, a front pipe, and the like. To
connect these components, coupling components called flanges are
often used. For automobile exhaust components, flange coupling is
positively employed because the flange coupling reduces working
hours for work as well as spaces for work.
From the viewpoint of preventing noise caused by vibration and
ensuring rigidity, thick flanges having thicknesses of 5 mm or more
are often used. Flanges are produced through processes such as
punching and press forming, and a steel sheet made of a
conventional common steel has been used as a starting material of
flanges. However, flanges made of a common steel, which are poor in
corrosion resistance as compared with other exhaust components made
of a stainless steel, shows rust, which in some cases mar their
appearance. Hence, in place of common steel sheets, stainless steel
sheets have been positively employed as starting materials of
flanges.
A ferritic stainless steel has a low toughness as compared with a
common steel because the ferritic stainless steel contains Cr and
is difficult to refine its steel micro-structure through phase
transformation. In particular, a stainless steel containing high
Cr, Al, and Si has a problem of its low toughness, and therefore
measures such as heating a coil of a stainless steel before causing
the stainless steel to run and reducing a thickness of a hot-rolled
steel sheet.
To produce a hot-rolled steel sheet or a hot-rolled-annealed steel
sheet made of a ferritic stainless steel having a sheet thickness
of 5 mm or more, an increase in the sheet thickness further
degrades its toughness. A steel sheet, when uncoiled, is prone to
sheet breakage through a leveling process, a cutting process, an
annealing process of a hot-rolled steel sheet, a pickling process,
and the like. To pass a steel sheet through the above processes, it
is often necessary to connect coils by welding. However, an
increased plate thickness extends a time taken for the welding,
which causes a decrease in temperature of heated coil and may bring
about a brittle breakage. In a case of being in need of a steel
sheet made of a ferritic stainless steel having a sheet thickness
of more than 5 mm, it has been a conventional practice to produce
the steel sheet as a steel plate, which raises a problem in that
its production costs are high as compared with a case where the
steel sheet is produced as a heat rolled coil.
There have been a plurality of ideas presented for solving the
problem relating to toughness of ferritic stainless steel
sheet.
For example, JP60-228616A (Patent Document 1) discloses a producing
method for obtaining a high-purity ferritic-stainless-steel-based
hot-rolled steel strip that is so excellent in toughness that any
trouble, such as cracking, associated with cold uncoiling, cold
rolling, and various handlings is less likely to occur, in the
method, immediately after subjected to hot rolling, a steel strip
is rapidly cooled at a cooling rate of 10.degree. C./sec or more
and coiled at a temperature of 450.degree. C. or lower. Patent
Document 1 describes that the technique decreased impact fracture
transition temperature to -20.degree. C. or less, and describes by
way of its examples whether each of coils having a sheet thickness
of 3 mm was successfully uncoiled. Patent Documents 1 describes
that this technique makes it possible to avoid employing a
producing method that leads to large variations in toughness value
of hot-rolled steel strips, such as immersing hot-rolled steel
strips in a water tank to subject them to water cooling.
JP8-199237A (Patent Document 2) describes a method for producing a
hot-rolled steel strip having a sheet thickness of 4.5 mm or more
and 9.0 mm or less from a ferritic stainless steel that contains
0.20% to 0.80% of Nb and Cr: more than 13.5% to 15.5% and that is
excellent in low-temperature toughness when formed into a
hot-rolled steel sheet, in which, immediately after subjected to
hot rolling at 800.degree. C. or more, a steel strip is cooled and
coiled at a temperature that satisfies a relation of
t.times.T.ltoreq.3600, where t denotes a sheet thickness of the
hot-rolled steel strip and T denotes a coiling temperature in the
hot rolling.
JP2012-140687A (Patent Document 3) discloses a hot-rolled coil and
a hot-rolled annealed coil made of a Ti-containing ferritic
stainless steel that has a toughness and a ductility enough to
consistently prevent a problem of cracking of materials in a line
through which an uncoiled hot-rolled coil runs, and has a sheet
thickness of 5 to 12 mm. As means for the prevention, Patent
Document 3 describes a producing method in which a coiling
temperature is set at 570.degree. C. or more, and a coil is
immersed in water after 5 minutes or more elapse from an end of
coiling and when a surface temperature of an outermost
circumference of the coil is 550.degree. C. or more, and the coil
is retained in the water for 15 minutes for more.
In contrast, JP2012-140688A (Patent Document 4) discloses a
hot-rolled coil and a hot-rolled annealed coil made of a
Nb-containing ferritic stainless steel that has a toughness and a
ductility enough to consistently prevent a problem of cracking of
materials in a line through which an uncoiled hot-rolled coil runs,
and has a sheet thickness of 5 to 10 mm. As means for the
prevention, Patent Document 4 describes a producing method in which
a stainless-steel slab is subjected to finish rolling at a rolling
finishing temperature of 890.degree. C. or more, water-cooled
before coiling, and coiled into a coil at a coiling temperature of
400.degree. C., and the coil is immersed into water within 30
minutes from an end of the coiling and retained in the water for 15
minutes for more.
JP2000-169943A (Patent Document 5) discloses a ferritic stainless
steel consisting of, in mass percent, C: 0.001 to 0.1%, N: 0.001 to
0.05%, Cr: 10 to 25%, S: 0.01% or less, P: 0.04% or less, Mn: 0.01
to 2%, Si: 0.01 to 2%, O: 0.01% or less, Sn: 0.05% to 2%, with the
balance being Fe and unavoidable impurities. Patent Document 5
describes that this ferritic stainless steel does not suffer aging
deterioration in its high temperature strength with time even in
long-time use at high temperature.
LIST OF PRIOR ART DOCUMENTS
Patent Document
Patent Document 1: JP60-228616A
Patent Document 2: JP8-199237A
Patent Document 3: JP2012-140687A
Patent Document 4: JP2012-140688A
Patent Document 5: JP2000-169943A
SUMMARY OF INVENTION
Technical Problem
For the technique of Patent Document 1, it is difficult to improve
a toughness of a thick ferritic stainless steel sheet having a
sheet thickness of more than 5 mm.
The technique of Patent Document 2 makes it possible to improve a
toughness of a Nb-added steel but fails to obtain an effect of
enhancing a toughness of a Ti-added steel.
The improvement in toughness of by subjecting a coil to water
cooling a coil, as with the technique of Patent Document 3, has a
problem of large fluctuations in cooling rate occurring in the
coil, which results in variations in toughness.
The technique of Patent Document 4 is directed to a Nb-containing
ferritic stainless steel, where a hot rolling finishing temperature
is set at 890.degree. C. or more, coiling is performed at
400.degree. C. or less, and the coil is immersed in water in order
to adjust hardness and a Charpy impact value; therefore, as stated
in Patent Document 1, a problem arises in that large fluctuations
in cooling rate occurs in the coil, which results in variations in
toughness.
The technique in Patent Document 5 includes performing hot rolling
with a heating temperature set at 1000.degree. C. or more and
1300.degree. C. or less, which therefore fails to reduce grain
sizes of a ferritic stainless steel sheet having a plate thickness
of more than 5 mm; therefore, it is difficult for the technique to
improve toughness.
An objective of the present invention is to solve problems of known
techniques and to produce a ferritic stainless steel sheet
excellent in toughness efficiently.
Solution to Problem
To solve the above problems, the present inventors conducted
detailed studies on a low-temperature toughness of a ferritic
stainless steel sheet from standpoints of components, hot-rolling
conditions and steel micro-structures, and clarified influences on
structure changes and toughness in the manufacturing process.
A titanium-added ferritic stainless steel does not experience phase
transformation in its manufacturing process, which makes it
difficult to control its steel micro-structure. That is, a slab to
be subjected to hot rolling has a plate thickness of 150 to 250 mm
and includes a steel micro-structure that is a solidification
structure, that is, a coarse columnar crystallite. The columnar
crystallite has a width of several hundred micrometers to ten-odd
millimeters and a length of several millimeters to several
centimeters. In the hot rolling, the slab is normally heated to
1100.degree. C. to 1300.degree. C. in a reheating furnace and
rolled by reversible rolling using a roughing mill into a sheet bar
having a plate thickness of 20 to 40 mm, when most parts of
structures recrystallize to be refined to several hundred
micrometers in terms of grain size. The sheet bar is rolled in a
subsequent finish hot rolling process to have a desired plate
thickness. The finish hot rolling is performed normally in a tandem
manner, in which rolling is performed in one direction, but in a
case of using Steckel mill, even the finish hot rolling is
performed in a reversible manner. In the finish hot rolling,
structures subjected to the rough hot rolling were only elongated
and expanded, and only very few of them experience
recrystallization.
The present inventor investigated changes occurring in structures
in the above processes and their influences on a material quality
and found, through the investigation, that refining
rough-hot-rolled structures is very effective to enhance a
toughness of a hot-rolled steel sheet. To refine a steel
micro-structure, performing severe plastic deformation at low
temperature is effective, but when hot rolling is performed at low
temperature, recrystallization after the hot rolling is also
delayed: therefore, after the rough hot rolling, unrecrystallized
portions tend to remain in structures in a rough bar immediately
before finish hot rolling. When the rough bar including the
remaining unrecrystallized portions is subjected to finish rolling
to be produced into a hot-rolled coil and the hot-rolled coil is
subjected to cold rolling annealing to be produced into a sheet,
the sheet shows coarse surface deterioration called ridging after
metal working; therefore, in conventional practices, hot rolling
with low temperature heating, which causes unrecrystallized
portions to remain in rough-hot-rolled structures, has been avoided
in production of a hot-rolled steel strip made of a ferritic
stainless steel.
In contrast, as a steel product for a flange as automobile exhaust
component, a common steel has been used in conventional practices;
however, in recent years, a ferritic stainless steel, which has a
high corrosion resistance, has been used. The above flange needs a
certain level of thickness but is not needed to have a very high
surface texture, and therefore, a steel plate made of a ferritic
stainless steel is mainly used. To enhance productivity, it is
preferable to use a hot coil made of a ferritic stainless steel.
However, the hot coil is needed to have an excellent toughness so
as to prevent a breakage from occurring when the hot coil is
uncoiled or runs through a leveling process and a pickling process.
The toughness tends to decrease particularly as the sheet thickness
increases.
Hence, the present inventors conducted studies and found that a
toughness of a hot-rolled steel sheet and a toughness of a
hot-rolled-annealed steel sheet are enhanced by performing grain
refinement on most of structures in a rough bar even when
unrecrystallized portions remain in the rough bar. To refine the
rough-hot-rolled structures, it is important to set a heating
temperature of hot rolling at 940 to 990.degree. C. and to perform
a rough-hot-rolling process at a temperature as low as possible.
However, an excessively lowered the heating temperature makes it
difficult to bring about the recrystallization during a period from
the rough-hot-rolling process to a start of finish hot rolling. It
is therefore particularly important to inhibit a decrease in
temperature of a steel strip during the period from the end of
rough hot rolling to the start of finish hot rolling. For flange
coupling parts, a steel sheet that is not subjected to cold rolling
but subjected to hot rolling, and therefore, there is no problem of
the ridging in the first place.
The left side of FIG. 1 is an enlarged view of a microstructure of
an example of a steel product according of the present invention,
and the right side is an enlarged view of a microstructure of a
conventional steel product, and comparison between them shows that
the steel product according to the present invention is made up of
fine grain structures, and the steel product according to the
present invention provides an absorbed energy value in the Charpy
impact test of 40 J/cm.sup.2 or more, whereas the conventional
steel product shows about 20 J/cm.sup.2 or less.
The gist of the present invention to solve the problems described
above is as follows.
(1) A ferritic stainless steel sheet having a sheet thickness t of
5.0 to 12.0 mm, including
a chemical composition consisting of, in mass percent:
C: 0.001 to 0.010%;
Si: 0.01 to 1.0%;
Mn: 0.01 to 1.0%;
P: 0.04% or less;
S: 0.010% or less;
Cr: 10.0 to 20.0%;
Ni: 0.01 to 1.0%;
Ti: 0.10 to 0.30%;
V: 0.01 to 0.40%;
Al: 0.005 to 0.3%;
N: 0.001 to 0.02%;
B: 0 to 0.0030%;
Mo: 0 to 2.0%;
Cu: 0 to 0.3%;
Mg: 0 to 0.0030%;
Sn: 0 to 0.1%;
Sb: 0 to 0.1%;
Zr: 0 to 0.1%;
Ta: 0 to 0.1%;
Nb: 0 to 0.1%;
Hf: 0 to 0.1%;
W: 0 to 0.1%;
Co: 0 to 0.2%;
Ca: 0 to 0.0030%;
REM: 0 to 0.05%; and
Ga: 0 to 0.1%,
with the balance being Fe and unavoidable impurities, wherein
in a steel micro-structure, on a cross section parallel to a
rolling direction, an area ratio of structures each satisfying:
major grain diameter/minor grain diameter being 5.0 or more is 90%
or more, and an average minor grain diameter of the structures is
100 .mu.m or less.
(2) A hot coil made of the ferritic stainless steel sheet according
to the above (1).
(3) An automobile exhaust flange member made of the ferritic
stainless steel sheet according to the above (1).
(4) An automobile exhaust flange member made using the ferritic
stainless hot coil according to the above (2).
Advantageous Effects of Invention
According to the present invention, it is possible to provide
efficiently a ferritic stainless steel sheet excellent in
toughness. The ferritic stainless steel sheet is particularly
suitable to an automobile exhaust flange member.
BRIEF DESCRIPTION OF DRAWINGS
FIG. 1 is a diagram illustrating a microstructure of a steel
product according to the present invention and a microstructure of
a conventional steel product.
FIG. 2 is a graph illustrating influences of average minor grain
diameter on Charpy impact value at 25.degree. C.
DESCRIPTION OF EMBODIMENTS
1. Chemical Composition
C: 0.001 to 0.010%
C (carbon) degrades toughness through hardening brought by
dissolved C and through precipitation in a form of carbides;
therefore, the smaller a content of C is, the better it is. An
excessive content of C causes deterioration in toughness
attributable to the formation of the carbides; therefore, an upper
limit of the content of C is set at 0.010%. Excessive reduction in
C however leads to increase in refining costs; therefore, a lower
limit of the content of C is set at 0.001%. In addition, in
consideration of production costs, corrosion resistance, and a
toughness of the steel sheet, the lower limit may be set at 0.002%
or 0.003%, and the upper limit may be set at 0.009%, 0.008%, or
0.007%.
Si: 0.01 to 1.0%
Si (silicon) may be added as a deoxidizing element and, in
addition, enhances oxidation resistance; however, from a viewpoint
of toughness; the smaller a content of Si is, the better it is
because Si is a solid-solution strengthening element. An excessive
content of Si causes significant deterioration in toughness, and
therefore, an upper limit of the content of Si is set at 1.0%.
Meanwhile, to ensure an oxidation resistance, a lower limit of the
content of Si is set at 0.01%. Excessive reduction in Si however
leads to increase in refining costs; therefore, in consideration of
material quality, initial rust resistance, and the like, the lower
limit may be set at 0.05, 0.10%, or 0.15%, and the upper limit may
be set at 0.9%, 0.8%, 0.7%, or 0.6%.
Mn: 0.01 to 1.0%
Mn (manganese) is, as with Si, a solid-solution strengthening
element, and therefore, in consideration of material quality, the
smaller a content of Mn is, the better it is. In particular, an
excessive content of Mn delays recrystallization caused by
precipitation of y phases during hot rolling, which may degrade
toughness; therefore, an upper limit of a content of Mn is set at
1.0%. Meanwhile, excessive reduction in Mn leads to increase in
refining costs, and in addition, addition of a minute quantity of
Mn enhances scale peeling property; therefore, a lower limit of the
content of Mn is set at 0.01%. In addition, in consideration of
material quality, production costs, and the like, the lower limit
may be set at 0.1%, 0.2%, 0.25%, or 0.3%, and the upper limit may
be set at 0.7%, 0.6%, 0.5%, or 0.4%.
P: 0.04% or less
P (phosphorus) is an element that is mixed in the steel sheet in a
form of an unavoidable impurity from raw material, such as
ferrochrome, and has a solid-solution strengthening capability
stronger than those of Mn and Si. For a purpose of hardening a
material, the smaller a content of P is, the better it is, from a
viewpoint of toughness. An excessive content of P causes
embrittlement attributable to grain-boundary segregation of P;
therefore, an upper limit of the content of P is set at 0.04%. A
lower limit of the content of P is not needed to be determined
particularly and is 0%. Excessive reduction in P however leads to
increase in raw-material costs, and therefore a lower limit of the
content of P may be set at 0.005%, 0.01%, or 0.015%. In addition,
in consideration of corrosion resistance, the upper limit may be
set at 0.03%, 0.025%, or 0.02%.
S: 0.010% or less
S (sulfur) is also an element mixed in the steel sheet in a form of
an unavoidable impurity and degrades corrosion resistance;
therefore, the smaller a content of S is, the better it is. An
excessive content of S tends to delay recrystallization in rough
hot rolling attributable to formation of precipitations such as
MnS, Ti.sub.4C.sub.2S.sub.2; therefore, an upper limit of the
content of S is set at 0.010%. A lower limit of the content of S is
not needed to be determined particularly and is 0%. S, however,
combines with Mn or Ti to exert an effect of enhancement in
punching property in flange forming. To obtain this effect, a lower
limit of the content of S may be set at 0.0002%, 0.0005%, or
0.001%. In addition, in consideration of inhibition of crevice
corrosion when the steel sheet is used as a fuel-system part, the
upper limit may be set at 0.008%, 0.006%, or 0.005%.
Cr: 10.0 to 20.0%
Cr (chromium) is an element that enhances corrosion resistance and
oxidation resistance, and in consideration of a salt corrosion
resistance required of a flange, it is necessary to contain Cr at
10.0% or more. Meanwhile, an excessive content of Cr makes the
steel sheet hard, degrading formability and toughness. In addition,
Cr tends to delay recrystallization in rough hot rolling in a form
of dissolved Cr, and when a content of Cr is more than 20.0%,
unrecrystallized structures remains immediately before finish hot
rolling to degrade toughness of the steel sheet; therefore, an
upper limit of the content of Cr is set at 20.0%. In consideration
of production costs, breakage of the steel sheet in production due
to deterioration in toughness, and the like, a lower limit of the
content of Cr may be set at 11.0%, 12.0%, or 13.0%. The upper limit
may be set at 19.0%, 18.0%, or 17.0%
Ni: 0.01 to 1.0%
Ni (nickel) inhibits crevice corrosion, and enhances initial rust
resistance by promoting repassivation; therefore 0.01% or more of
Ni is contained. An excessive content of Ni however leads to
hardening, degrading formability, and promotes precipitation of
austenite phases during hot rolling, delaying recrystallization
during rough hot rolling, and in addition, causes stress corrosion
cracking to occur easily; therefore, an upper limit of a content of
Ni is set at 1.0%. In addition, in consideration of raw-material
costs and the like, a lower limit of the content of Ni may be set
at 0.02%, 0.03%, or 0.05%, and the upper limit may be set at 0.5%,
0.3%, 0.2%, or 0.1%.
Ti: 0.10 to 0.30%
Ti (titanium) is an element that is added to enhance corrosion
resistance, intergranular corrosion resistance, and toughness by
combining with C, N, S, and P. In particular, if C and N are not
immobilized sufficiently, sensitization occurs to form a Cr
depleted zone, resulting in a significant deterioration in
corrosion resistance; therefore, a lower limit of a content of Ti
is 0.10%.
To ensure a corrosion resistance of the steel sheet as well as its
weld zone, the lower limit may be set at 0.12%, 0.14%, or 0.16%.
Meanwhile, an excessive content of Ti causes coarse TiN to
precipitate in molten steel in a steelmaking process, degrading a
toughness of the steel sheet; therefore, an upper limit of the
content of Ti is set at 0.30%. In consideration of production costs
and the like, the upper limit may be set at 0.28%, 0.25%, or
0.22%
V: 0.01 to 0.40%
V (vanadium) inhibits crevice corrosion, and in addition,
contributes to enhancement in toughness when added in minute
quantity; therefore 0.01% or more of V is contained. An excessive
content of V however leads to hardening, degrading formability, and
in addition, causes coarse V(C, N) to precipitate, causing
deterioration in toughness; therefore, an upper limit of a content
of V is set at 0.4%. In consideration of the enhancement in
toughness, raw-material costs, initial rust resistance, and the
like, a lower limit of the content of V may be set at 0.02%, 0.03%,
or 0.04%, and the upper limit may be set at 0.20%, 0.10%, or
0.06%.
Al: 0.005 to 0.3%
Al (aluminum) is an element added as a deoxidizing element and
enhances a toughness of the steel sheet by reducing oxides in the
steel. Al exerts the action when a content of Al is 0.005% or more,
and therefore, a lower limit of the content of Al is set at 0.005%.
An excessive content of Al causes deterioration in toughness and
degradation in weldability and surface quality, and in addition
delays recrystallization in rough hot rolling; therefore, an upper
limit of the content of Al is 0.3%. In addition, in consideration
of refining costs and the like, the lower limit may be set at
0.01%, 0.02%, or 0.03%, and the upper limit may be set at 0.15%,
0.1%, 0.08%, or 0.06%.
N: 0.001 to 0.02%
N (nitrogen) degrades toughness and corrosion resistance as with C,
and the smaller a content of N is, the better it is. An excessive
content of N causes deterioration in toughness attributable to
formation of coarse nitrides, which brings about a situation where
improvement in toughness cannot be achieved only by refining grain
sizes; therefore, an upper limit of the content of N is set at
0.02%. Excessive decrease in N however leads to increase in
refining costs; therefore, a lower limit of the content of N is set
at 0.001%. In addition, in consideration of production costs,
workability, initial rust resistance, and the like, a lower limit
of the content of N may be set at 0.003%, 0.005%, or 0.006%, and
the upper limit may be set at 0.015%, 0.010%, or 0.009%.
Although N is preferably reduced from a viewpoint of enhancing a
toughness of a ferritic stainless steel, it is also useful, from a
viewpoint of corrosion resistance, oxidation resistance, pressing
formability, and reducing hot rolling flaws, to add a proper amount
of at least one of B, Mo, Cu, Mg, Sn, Sb, Zr, Ta, Nb, W, Co, Ca,
REM, Ga, and Bi.
B: 0 to 0.0030%
B (boron) is an element that enhances secondary metal workability
of a product by segregating in grain boundaries and therefore may
be contained to enhance a punching property of a flange. An
excessive content of B however causes borides to precipitate,
degrading toughness, and in addition, delays recrystallization
during rough hot rolling; therefore, an upper limit of a content of
B is set at 0.0030%. A lower limit of the content of B is not
needed to be determined particularly and is 0%. For enhancement in
toughness and the like, the lower limit may be set at 0.0001% or
0.0002%. In consideration of costs and deterioration in ductility,
the upper limit may be set at 0.0020%, 0.0010%, or 0.0005%.
Mo: 0 to 2.0%
Mo (molybdenum) is an element that enhances corrosion resistance
and high-temperature strength, and in particular, in a case of
having a crevice structure, Mo may be contained to inhibit crevice
corrosion. An excessive content of Mo increases oxidation
resistance significantly, causing a flow during heating for hot
rolling due to breakaway oxidation, and delays recrystallization in
rough hot rolling to coarsen rough-hot-rolled structure, causing
deterioration in toughness; therefore, an upper limit of a content
of Mo is set at 2.0%. A lower limit of the content of Mo is not
needed to be determined particularly and is 0%. For enhancement in
toughness and the like, 0.01% of Mo may be contained. In addition,
in consideration of production costs and the like, the lower limit
may be set at 0.02% or 0.03%, and the upper limit may be set at
1.2%, 0.3%, or 0.1%.
Cu: 0 to 0.3%
Cu (copper) may be contained because Cu enhances high-temperature
strength, and in addition, inhibits crevice corrosion and promotes
repassivation. An excessive content of Cu leads to hardening by
precipitation of .epsilon.-Cu and Cu-rich clusters, degrading
formability and toughness; therefore, an upper limit of a content
of Cu is set at 0.3%. A lower limit of the content of Cu is not
needed to be determined particularly and is 0%. For enhancement in
formability and toughness, 0.01% or more of Cu may be contained. In
consideration of pickling property in production, the lower limit
may be set at 0.01% or 0.03%, and the upper limit may be set at
0.02%, 0.12%, or 0.10%.
Mg: 0 to 0.0030%
Mg (magnesium) is in some cases added as a deoxidizing element and
in addition, is an element that contributes to enhancement in
formability by refining structures of a slab. In addition, a Mg
oxide serves as a precipitation site for carbo-nitrides such as
Ti(C, N) and Nb(C, N) and has an effect of fine dispersing
precipitation of these carbo-nitrides. For that reason, Mg may be
contained. An excessive content of Mg however leads to
deterioration in weldability and corrosion resistance; therefore,
an upper limit of a content of Mg is set at 0.0030%. A lower limit
of the content of Mg is not needed to be determined particularly
and is 0%. The lower limit may be set at 0.0003%, 0.0006%, or 0.01%
as necessary. In consideration of refining costs and the like, the
upper limit may be set at 0.0020% or 0.0010%.
Sn: 0 to 0.1%
Sb: 0 to 0.1%
Sn (tin) and Sb (antimony) may be contained because Sn and Sb
contribute to enhancement in corrosion resistance and high
temperature strength. Excessive contents of Sn and Sb cause slab
cracking in production of the steel sheet, and in addition, cause
deterioration in a toughness of the steel sheet; therefore, upper
limits of contents of Sn and Sb are set at 0.1%. Lower limits of
contents of Sn and Sb are not needed to be determined particularly
and are 0%. The lower limits may be set at 0.005% or 0.01% as
necessary. In addition, in consideration of refining costs,
producibility, and the like, the upper limits may be set at 0.05%
or 0.02%.
Zr: 0 to 0.1%
Ta: 0 to 0.1%
Nb: 0 to 0.1%
Hf: 0 to 0.1%
Zr (zirconium), Ta (tantalum), Nb (niobium), or Hf (hafnium) may be
contained because Zr, Ta, Nb, and Hf combine C and N to contribute
to enhancement in toughness. Excessive contents of Zr, Ta, Nb, and
Hf however increase costs and in addition, cause large
carbo-nitrides to precipitate, degrading a toughness of the steel
sheet significantly; therefore, upper limits of contents of Zr, Ta,
Nb, and Hf are set at 0.1%. Lower limits of contents of Zr, Ta, Nb,
and Hf are not needed to be determined particularly and are 0%. The
lower limits may be set at 0.005% or 0.01% as necessary. In
addition, in consideration of refining costs, producibility, and
the like, the upper limits may be set at 0.08% or 0.03%.
W: 0 to 0.1%
As with Mo, W (tungsten) may be contained because W contributes to
enhancement in corrosion resistance and high temperature strength.
An excessive content of W leads to deterioration in toughness and
increase in costs in production of the steel sheet; therefore, an
upper limit of a content of W is set at 0.1%. A lower limit of the
content of W is not needed to be determined particularly and is 0%.
The lower limit may be set at 0.01% as necessary. In consideration
of refining costs, producibility, and the like, the upper limit may
be set at 0.05% or 0.02%.
Co: 0 to 0.2%
Co (cobalt) may be contained because Co contributes to enhancement
in high temperature strength. An excessive content of Co causes
deterioration in toughness due to solid-solution strengthening or
inhibit of recrystallization during rough hot rolling; therefore,
an upper limit of a content of Co is set at 0.2%. A lower limit of
the content of Co is not needed to be determined particularly and
is 0%. To obtain this effect, the lower limit may be set at 0.01%,
0.02%, or 0.04%. In addition, in consideration of refining costs,
producibility, and the like, the upper limit may be set at 0.15% or
0.1%.
Ca: 0 to 0.0030%
Ca (calcium) may be contained because Ca has a desulfurizing
effect. An excessive content of Ca however causes formation of
coarse CaS, degrading corrosion resistance; therefore, an upper
limit of a content of Ca is set at 0.0030%. A lower limit of the
content of Ca is not needed to be determined particularly and is
0%. In consideration of refining costs, producibility, and the
like, the upper limit may be set at 0.0030% or 0.0020%.
REM: 0 to 0.05%
REM may be contained because REM has an effect of enhancing
toughness by refining various precipitates and has an effect of
enhancing oxidation resistance. An excessive content of REM however
makes castability significantly poor and in addition, degrades
toughness through solid-solution strengthening and by inhibiting
recrystallization in rough hot rolling; therefore, an upper limit
of a content of REM is set at 0.05%. A lower limit of the content
of REM is not needed to be determined particularly and is 0%. To
obtain this effect, the lower limit may be set at 0.001% or 0.002%.
In addition, in consideration of refining costs, producibility, and
the like, the upper limit may be set at 0.01% or 0.005%. According
to a common definition, REM (rare earth metal) refers to a generic
term for 2 elements, scandium (Sc), yttrium (Y), and 15 elements
(lanthanoid), from lantern (La) through lutetium (Lu). One element
of REM may be added, or mixture of elements of REM may be
added.
Ga: 0 to 0.1%
Ga (gallium) may be contained at a content within a range of 0.1%
or less for enhancement in corrosion resistance and inhibition of
hydrogen embrittlement. A lower limit of a content of Ga is not
needed to be determined particularly and is 0%. The lower limit may
be set at 0.0002% as necessary, from a viewpoint of formation of
its sulfide and its hydride. An upper limit of the content of Ga
may be set at 0.0020% from a viewpoint of producibility and costs
and a viewpoint of promotion of recrystallization in rough hot
rolling.
Components other than those described above are not specifically
defined in the present invention, but in the present invention,
0.001 to 0.1% of Bi or the like may be contained as needed. Note
that commonly harmful elements and impurity elements such as As and
Pb are preferably reduced as much as possible.
2. Steel Micro-Structure
In a steel micro-structure of the ferritic stainless steel sheet
according to the present invention, an area ratio of structures
each satisfying: major grain diameter/minor grain diameter being
5.0 or more is 90% or more in a cross section of the steel sheet
parallel to a rolling direction. The area ratio of the structures
each satisfying: major grain diameter/minor grain diameter being
5.0 or more being 90% or more means that the ferritic stainless
steel sheet according to the present invention is a steel sheet as
it is after hot rolling. The area ratio of the above structures is
preferably made as high as possible. A lower limit of the area
ratio may be set at 91%, 92%, or 93% as necessary. An upper limit
of the area ratio is 100% but may be set at 99% or 98%. Here,
measurement of the steel micro-structure is performed in such a
manner that grain boundaries are exposed on a cross section of the
steel sheet parallel to the rolling direction and a sheet-thickness
direction by nitric-acid electrolytic etching, a zone having at
least 1 mm.sup.2 is observed under an optical microscope at
positions of 0.25 t (t: sheet thickness) and 0.50 t (t: sheet
thickness), and an area fraction of grains each of which a ratio of
a major grain diameter and a minor grain diameter (major grain
diameter/minor grain diameter) is 5.0 or more is measured. A
reference of the structures each having a major grain
diameter/minor grain diameter being 5.0 or more is that an average
value of the area fraction at the 0.25 t position and the 0.50 t
position is 90% or more.
An average minor grain diameter of the ferritic stainless steel
sheet according to the present invention is 100 .mu.m or less.
Here, an average minor grain diameter at 0.25 t to 0.75 t (t: plate
thickness) is used as a reference. Specifically, the "average minor
grain diameter" is determined in such a manner that grain
boundaries are exposed on the cross section of the steel sheet
parallel to the rolling direction and the sheet-thickness direction
by nitric-acid electrolytic etching, and a line parallel to the
sheet thickness direction is observed within a range of 0.25 t to
0.75 t (t: sheet thickness), a number of grains captured on the
line is measured to JIS G0551 Appendix C.2, and an actual length of
the length is divided by the number of grains.
As illustrated in FIG. 2, an average minor grain diameter being
more than 100 .mu.m yields a low Charpy impact value at 25.degree.
C. However, an average minor grain diameter being 100 .mu.m or less
increases a Charpy impact value at 25.degree. C. to 40 J/cm.sup.2
or more, results in enhancement in a toughness of the steel sheet.
At this point, even if coarse, elongated and expanded grains that
are unrecrystallized during rough hot rolling remain partially, a
toughness required for the steel sheet is ensured by fine,
elongated and expanded ferrite grains surrounding the coarse,
elongated and expanded grains. For that reason, an upper limit of
the average minor grain diameter is set at 100 .mu.m. For
enhancement in toughness, the upper limit may be set at 95 .mu.m,
90 .mu.m, 85 .mu.m, 80 .mu.m, or 78 .mu.m. In contrast, when severe
plastic deformation at low temperature is performed to refine
structures, galling is likely to be caused between the steel sheet
and a rolling work roll in the hot rolling, which also limits the
refining of the structures; therefore, a lower limit of the average
minor grain diameter may be set at 30 .mu.m. The lower limit may be
set at 40 .mu.m, 47 .mu.m, 51 .mu.m, 55 .mu.m, or 60 .mu.m as
necessary.
3. Producing Method
The steel sheet according to the present invention is produced by a
steelmaking process and hot rolling.
There is no special limitation on the steelmaking process. For
example, a preferable method is one in which steels having the
chemical composition described above is melted in a converter,
followed by second refining. The melted molten steel is formed into
slabs in conformity with a known casting method (continuous
casting). The slabs are heated to a predetermined temperature and
subjected to hot rolling by continuous rolling, so as to have a
predetermined sheet thickness.
The hot rolling process is a particularly important process to
obtain the steel micro-structure according to the present
invention. The present inventors have confirmed through previously
conducted studies that the steel micro-structure according to the
present inventors can be obtained in a case where the following
recommended conditions are satisfied.
(a) Heating Temperature: 940 to 990.degree. C.
To make rough-hot-rolled structures fine, a heating temperature
needs to be lowered and is set at 990.degree. C. or less. An
excessively low heating temperature however may cause hot rolling
flaws; therefore, the heating temperature is set at 940.degree. C.
or more.
(b) Rough-Hot-Rolling Entrance-Side Temperature: 900 to 950.degree.
C.
By setting an entrance side temperature in rough hot rolling at
950.degree. C. or less, the rough-hot-rolled structures can be
refined. Even when the heating temperature is high, a
rough-hot-rolling starting temperature can be lowered by cooling a
slab by a time of the rough hot rolling. However, excessively
lowering the entrance-side temperature causes hot rolling flaws;
therefore, the entrance-side temperature is set at 900.degree. C.
or more.
(c) Rough-Hot-Rolling Ending Temperature: 850 to 900.degree. C.
When a rough-hot-rolling ending temperature is more than
900.degree. C., rough-hot-rolled structures are coarsened. In
contrast, when the rough-hot-rolled ending temperature falls below
850.degree. C., recrystallization after the rough hot rolling is
delayed, which coarsens the rough-hot-rolled structures (structures
immediately before finish hot rolling), degrading a toughness of a
hot-rolled sheet after the finish hot rolling. For that reason, the
rough-hot-rolling ending temperature is set at 850 to 900.degree.
C. Note that the rough-hot-rolling ending temperature is
substantially determined depending on the rough hot rolling
starting temperature. However, the rough-hot-rolling ending
temperature can be lowered by increasing a number of passes of the
rough hot rolling or increasing a rolling reduction of the rough
hot rolling.
(d) Rough Rolling Reduction: 80% or More
By setting a rolling reduction of the rough hot rolling at 80% or
more, the rough-hot-rolled structures can be refined. An upper
limit of the rolling reduction of the rough hot rolling are not
needed to be determined specifically, but in actual production, the
rolling reduction seldom becomes more than 95%; therefore, the
upper limit may be set at 95%.
(e) Bar Heater: Temperature Rise of 30.degree. C. or More
The rough hot rolling is performed as reversible rolling, and
finish hot rolling is performed as unidirectional rolling using a
tandem hot rolling mill. For that reason, a rough hot rolling mill
and a finish hot rolling mill are separated from each other by a
space of about 100 m, through which a temperature of a sheet bar
decreases greatly. If the decrease in temperature occurring in the
space is excessive, a load of the finish hot rolling becomes heavy,
which makes quality unstable and in addition, fails to bring the
steel micro-structure into a desired state. Moreover, the excessive
decrease in temperature increases a ratio of unrecrystallized
structures, increasing an average grain size. For that reason, a
finish-hot-rolling starting temperature of a hot-rolled coil needs
to be uniform in a longitudinal direction of the coil. It is
therefore important to use a bar heater of an induction system to
heat a sheet bar (rough bar). It is necessary for a ferritic
stainless steel not to experience phase transformation and to
refine solidification structures of a slab through
recrystallization after the rough hot rolling; however, in order to
perform the recrystallization by means of strains brought by the
rough hot rolling, using a bar heater to prevent the decrease in
temperature after the rough hot rolling is effective. Specifically,
the bar heater is used to bring about a temperature rise of
30.degree. C. or more. In contrast, an excessive temperature rise
causes grain growth coarsening the rough-hot-rolled structures;
therefore, the temperature rise is preferably set at 55.degree. C.
or less.
(f) Heat Insulation Cover: Heat Conservation
Similarly to using the bar heater, as a method to inhibit the
decrease in temperature of the sheet bar, heat insulation covers
are provided on surfaces sandwiching vertically a conveyance table
provided between the rough hot rolling and the finish hot rolling
to perform heat conservation, by which structure refining through
recrystallization is intended.
(g) Finish-Hot-Rolling Entrance-Side Temperature: 840 to
890.degree. C.
In a finish hot rolling process, a sheet bar having a sheet
thickness of 28 to 38 mm is rolled to have a required hot-rolled
sheet thickness, so that rough-hot-rolled structures are elongated
and expanded, by which strains are accumulated. In this process, by
accumulating strains in a large amount, a toughness of a hot-rolled
sheet can be enhanced. To accumulate the strains (increase a
dislocation density), a rolling starting temperature is set at
890.degree. C. or less, but an excessively lowered rolling starting
temperature causes hot rolling flaws. For that reason, a
finish-hot-rolling entrance-side temperature is set at 840 to
890.degree. C.
(h) Finish-Hot-Rolling Ending Temperature: 690 to 740.degree.
C.
Similarly to the finish-hot-rolling starting temperature, when a
finish-hot-rolling ending temperature is lowered, strains are
accumulated, increasing toughness, but an excessively lowered
finish-hot-rolling ending temperature causes hot rolling flaws. The
cause of hot rolling flaws described herein is mainly galling
between the hot rolling work roll and the hot-rolled sheet. For
that reason, the finish-hot-rolling ending temperature is set at
690 to 740.degree. C. Note that the finish-hot-rolling ending
temperature is determined in conjunction with the
finish-hot-rolling starting temperature starting temperature but is
changed depending on a rolling speed and the sheet thickness.
(i) Finish Rolling Reduction: 60% or More
By setting a rolling reduction of the finish rolling at 60% or
more, the rough-hot-rolled structures can be refined. An upper
limit of the rolling reduction of the finish rolling does not be
determined specifically, but in actual production, the rolling
reduction seldom becomes more than 95%; therefore, the upper limit
may be set at 95%.
(j) Allowed Period to Start Water Cooling: within 2 Seconds
Since a ferritic stainless steel does not experience phase
transformation, structures after the rough hot rolling is elongated
and expanded grains that are recrystallized grains produced by the
rough hot rolling are elongated and expanded by the finish hot
rolling. In order for the strains accumulated in the finish hot
rolling not to decrease due to recovery or recrystallization, the
steel sheet is cooled immediately after the finish hot rolling. A
period from an end of the finish hot rolling to a start of water
cooling is set at a period within 2 seconds.
(k) Cooling Rate: 25.degree. C./s or More
After the finish hot rolling, the hot-rolled sheet needs to be
cooled to an intended coiling temperature. The hot-rolled sheet
needs to be cooled to the intended coiling temperature between a
final stand of the finish hot rolling to a coiling machine
(coiler). At this point, the cooling is performed at a cooling rate
of 25.degree. C./s or more.
(1) Water Cooling Ending Temperature: 510 to 560.degree. C.
To control the coiling temperature, it is necessary to measure a
temperature of a hot-rolled sheet online using a radiation
thermometer or the like; however, when the temperature of the sheet
decreases to about 450.degree. C., water on a top of the sheet does
not evaporate but remain until the sheet reaches the coiler, which
makes it difficult to measure the temperature of the sheet;
therefore, a water cooling ending temperature is set at 510.degree.
C. or more. In order to decrease the coiling temperature to
550.degree. C. or less, the water cooling ending temperature is set
at 560.degree. C. or less.
(m) Coiling Temperature: 500 to 550.degree. C.
When the coiling temperature is excessively high, the strains
introduced in the finish hot rolling may decrease through recovery
or recrystallization, or precipitates such as FeTiP may precipitate
to degrade toughness. For that reason, the coiling temperature is
set at 550.degree. C. or less. However, when the coiling
temperature is excessively low, the measurement and control of the
temperature becomes difficult; therefore, the coiling temperature
is set at 500.degree. C. or more.
The hot-rolled coil produced according to the present invention
dispenses with cooling the whole coil in a water tank, which
simplify the producing process. The thickness of the hot-rolled
steel sheet is set at 5 to 12 mm or less, which is employed
frequently for flanges, but when the steel sheet is thickened
excessively, a toughness of the steel sheet deteriorates extremely;
therefore, the thickness is desirably 5 to 10 mm.
Through pickling, skin-pass rolling, or surface grinding after the
hot rolling, the hot-rolled steel sheet can be made suitable for a
flange.
EXAMPLE
Steels having chemical compositions shown in Table 1 were melted,
cast into slabs, and the slabs are subjected to the hot rolling
coil to 5 to 15 mm to be formed into hot-rolled coils. Conditions
for the production are shown in Table 2 and Table 3.
TABLE-US-00001 TABLE 1 Steel Chemical Composition (mass %, Balance:
Fe and unavoidable impurities) No. C Si Mn P S Cr Ni Ti V Al N
Others 1 0.005 0.45 0.35 0.027 0.001 11.1 0.02 0.20 0.03 0.02 0.008
2 0.005 0.12 0.35 0.025 0.001 17.1 0.01 0.18 0.04 0.02 0.006
0.0002% B 3 0.004 0.13 0.45 0.027 0.002 17.3 0.01 0.21 0.04 0.02
0.008 0.5% Mo 4 0.002 0.45 0.35 0.027 0.001 17.3 0.02 0.20 0.02
0.05 0.008 0.01% Sn, 0.01% Sb 5 0.004 0.62 0.35 0.017 0.002 17.3
0.02 0.21 0.02 0.05 0.008 0.01% Co 6 0.004 0.44 0.01 0.027 0.001
17.4 0.02 0.18 0.05 0.03 0.012 0.01% Cu, 0.1% Sb 7 0.005 0.42 1.00
0.020 0.001 17.3 0.30 0.21 0.01 0.04 0.006 0.1% Sn 8 0.004 0.12
0.12 0.010 0.002 17.2 0.02 0.22 0.02 0.03 0.001 1.2% Mo 9 0.002
0.11 0.45 0.040 0.001 17.3 0.01 0.23 0.05 0.05 0.007 0.3% Cu, 0.01%
W 10 0.005 0.01 0.12 0.026 0.0002 17.5 0.01 0.20 0.05 0.05 0.007
2.0% Mo 11 0.003 0.45 0.35 0.027 0.01 17.3 0.02 0.20 0.04 0.04
0.006 0.0030% B 12 0.010 0.12 0.12 0.030 0.001 10.0 0.07 0.22 0.05
0.04 0.020 0.0002% Mg, 0.1% Zr 13 0.006 0.10 0.12 0.027 0.002 20.0
0.30 0.10 0.03 0.04 0.008 0.0030% Mg, 0.1% Hf, 0.1% Ta, 0.1% W 14
0.001 0.90 0.35 0.025 0.003 17.4 0.02 0.10 0.04 0.04 0.006 0.0002%
Ga, 0.1% W 15 0.004 0.10 0.35 0.027 0.001 13.5 0.02 0.30 0.03 0.03
0.008 0.1% Co, 0.0030% Ca, 0.001% REM 16 0.005 1.00 0.10 0.025
0.002 17.3 0.08 0.20 0.02 0.05 0.006 0.0001% Ca, 0.1% Ga 17 0.004
0.11 0.35 0.025 0.004 17.5 0.11 0.10 0.40 0.05 0.007 0.01% Zr,
0.01% Ta 18 0.005 0.12 0.36 0.027 0.001 16.5 0.02 0.20 0.05 0.005
0.0012 19 0.005 0.46 0.10 0.029 0.001 18.1 0.01 0.40 0.03 0.30
0.007 0.01% Hf, 0.01% Nb 20 0.004 0.20 0.13 0.025 0.001 17.2 0.02
0.21 0.05 0.05 0.006 0.05% REM 21 0.012* 0.45 0.25 0.027 0.001 16.5
0.03 0.19 0.05 0.04 0.014 22 0.003 1.10* 0.45 0.026 0.001 17.2 0.01
0.18 0.03 0.03 0.008 23 0.004 0.45 1.10* 0.027 0.001 17.2 0.02 0.17
0.05 0.05 0.008 24 0.005 0.12 0.35 0.041* 0.001 18.1 0.01 0.21 0.03
0.03 0.006 25 0.006 0.15 0.12 0.027 0.011* 17.5 0.02 0.18 0.05 0.04
0.008 26 0.002 0.13 0.12 0.025 0.003 20.2* 0.02 0.25 0.03 0.05
0.008 27 0.004 0.14 0.24 0.025 0.001 17.1 1.10* 0.20 0.05 0.03
0.006 28 0.003 0.08 0.45 0.027 0.002 13.2 0.02 0.45* 0.03 0.04
0.007 29 0.002 0.45 0.23 0.025 0.003 17.5 0.01 0.20 0.50* 0.03
0.006 30 0.004 0.12 0.80 0.027 0.002 17.2 0.02 0.25 0.05 0.5* 0.006
31 0.003 0.13 0.21 0.025 0.001 17.2 0.01 0.21 0.03 0.03 0.025* 32
0.005 0.11 0.11 0.027 0.003 9.5* 0.01 0.21 0.03 0.04 0.007 0.0040%
B* 33 0.004 0.20 0.21 0.025 0.001 16.5 0.01 0.22 0.05 0.03 0.008
0.0050% Mg* 34 0.004 0.11 0.24 0.027 0.001 17.2 0.01 0.20 0.03 0.04
0.007 0.2% Sn* 35 0.004 0.11 0.00* 0.024 0.003 18.0 0.01 0.26 0.05
0.02 0.008 0.2% Sb* 36 0.004 0.10 0.12 0.025 0.001 11.2 0.01 0.20
0.00* 0.05 0.008 0.2% Zr* 37 0.006 0.30 0.25 0.024 0.001 17.2 0.01
0.22 0.05 0.04 0.007 0.2% Ta* 38 0.003 0.00* 0.13 0.025 0.001 17.2
0.01 0.18 0.05 0.03 0.008 0.2% Hf* 39 0.005 0.10 0.21 0.027 0.001
14.1 0.00* 0.19 0.03 0.05 0.006 2.5% W* 40 0.007 0.24 0.22 0.026
0.002 17.3 0.01 0.21 0.05 0.04 0.007 0.2% Co* 41 0.003 0.12 0.13
0.025 0.001 17.2 0.01 0.08* 0.03 0.03 0.006 0.0050% Ca* 42 0.003
0.23 0.21 0.025 0.002 17.5 0.01 0.18 0.05 0.002* 0.008 0.1% REM* 43
0.004 0.20 0.11 0.027 0.001 17.2 0.02 0.18 0.03 0.05 0.008 0.2% Ga*
44 0.004 1.00 0.35 0.026 0.001 17.3 0.02 0.21 0.01 0.05 0.008 The
mark "*" indicates that the value fell out of the range defined in
the present invention.
TABLE-US-00002 TABLE 2 ROUGH ROLLING TO FINISH ROLLING TEMPER-
ROUGH ROLLING ATURE HEATING STARTING ENDING RISE BY TEMPER- HEAT
SLAB TEMPER- TEMPER- TEMPER- ROLLING BAR ATURE CONSERVATION RUN
Steel THICKNESS ATURE ATURE ATURE REDUCTION HEATER RISE COVER
NUMBER No. (mm) (.degree. C.) (.degree. C.) (.degree. C.) (%) (Y/N)
(.degree. C.) (Y/N) INVENTIVE 1 1 250 980 950 850 88 Y 50 Y EXAMPLE
2 2 252 990 950 850 87 Y 50 Y 3 3 248 940 900 850 89 Y 50 Y 4 4 252
970 940 850 88 Y 50 Y 5 5 250 950 920 850 86 Y 50 Y 6 6 200 980 950
850 86 Y 30 Y 7 7 220 980 950 850 86 Y 40 Y 8 8 250 980 950 850 89
Y 50 Y 9 9 252 980 950 850 89 Y 50 Y 10 10 252 990 950 850 88 Y 50
Y 11 11 252 970 940 850 87 Y 50 Y 12 12 252 980 950 850 89 Y 30 Y
13 13 252 980 950 850 89 Y 50 Y 14 14 252 980 950 850 89 Y 50 Y 15
15 252 980 950 850 88 Y 50 Y 16 16 252 990 950 850 88 Y 30 Y 17 17
252 980 950 900 87 Y 50 Y 18 18 252 990 950 850 88 Y 40 Y 19 19 252
990 950 850 89 Y 50 Y 20 20 252 980 950 850 88 Y 50 Y COMPARATIVE 1
*21 250 1170 1140 1040 85 Y 50 Y EXAMPLE 2 *22 250 1170 1140 1040
85 Y 50 Y 3 *23 250 1170 1140 1040 85 Y 50 Y 4 *24 250 1250 1220
1120 85 Y 50 Y 5 *25 250 1170 1140 1040 85 Y 50 N 6 *26 250 1170
1140 1040 85 Y 50 Y 7 *27 250 1200 1170 1070 85 Y 50 Y 8 *28 250
1170 1140 1040 85 Y 50 Y 9 *29 200 1170 1140 1040 81 Y 50 N 10 *30
150 1190 1160 1060 75 Y 50 N 11 *31 250 1170 1140 1040 85 Y 50 Y 12
*32 251 1170 1140 1040 85 Y 50 N 13 *33 250 1170 1140 1040 85 Y 50
Y 14 *34 250 1220 1190 1090 85 Y 50 Y 15 *35 248 1170 1140 1040 85
Y 50 N 16 *36 250 1180 1150 1050 85 Y 50 Y 17 *37 250 1170 1140
1040 85 Y 50 Y 18 *38 250 1170 1140 1040 85 Y 50 N 19 *39 252 1170
1140 1040 85 Y 50 Y 20 *40 252 1170 1140 1040 85 Y 50 Y 21 *41 250
1190 1160 1060 85 Y 50 N 22 *42 250 1170 1140 1040 85 Y 50 Y 23 *43
250 1170 1140 1040 85 Y 50 Y 24 44 250 1170 1140 1040 85 Y 50 Y 25
44 250 1170 1140 1040 85 Y 50 Y 26 44 250 900 870 770 85 N 0 N 27
44 250 920 890 790 85 Y 50 Y 28 44 250 900 870 770 85 Y 50 Y The
mark "*" indicates that the value fell out of the range defined in
the present invention.
TABLE-US-00003 TABLE 3 FINISH ROLLING COOLING ENDING PERIOD STOP
COILING STARTING TEMPER- ROLLING TO COOLING TEMPER- TEMPERA- RUN
Steel TEMPERATURE ATURE REDUCTION THICKNESS START RATE ATURE ATURE
NUMBER No. (.degree. C.) (.degree. C.) (%) (mm) (s) (.degree. C./s)
(.degree. C.) (.degree. C.) INVENTIVE 1 1 850 705 73 8 1.5 62 550
550 EXAMPLE 2 2 860 715 65 12 1.5 66 550 550 3 3 840 690 64 10 1.5
72 510 500 4 4 840 695 73 8 1.5 58 550 550 5 5 850 690 76 8 1.5 58
545 545 6 6 840 690 71 8 1.5 56 550 550 7 7 840 695 77 7 1.5 58 550
550 8 8 850 705 71 8 1.5 78 510 510 9 9 850 705 79 6 1.5 62 550 550
10 10 860 715 73 8 1.5 66 550 550 11 11 840 695 76 8 1.5 58 550 550
12 12 840 690 71 8 1.5 72 510 510 13 13 850 705 71 8 1.5 58 560 550
14 14 850 705 71 8 1.5 62 550 550 15 15 850 705 83 5 1.5 78 510 510
16 16 840 690 73 8 1.5 56 550 550 17 17 850 705 76 8 1.5 62 550 550
18 18 850 705 73 8 1.5 78 510 510 19 19 860 715 71 8 1.5 66 550 550
20 20 870 730 73 8 1.5 83 523 523 COMPARATIVE 1 *21 1040 895 79 8
1.5 138 550 550 EXAMPLE 2 *22 1040 895 79 8 1.5 138 550 550 3 *23
1040 895 79 8 1.5 158 500 500 4 *24 1120 975 79 8 1.5 38 880 880 5
*25 1040 895 79 8 1.5 140 545 545 6 *26 1040 895 79 8 1.5 138 550
550 7 *27 1070 925 79 8 1.5 150 550 550 8 *28 1040 895 79 8 1.5 166
480 480 9 *29 1040 895 79 8 1.5 138 550 550 10 *30 1060 915 79 8
1.5 146 550 550 11 *31 1040 895 79 8 1.5 138 550 550 12 *32 1040
895 79 8 1.5 198 400 400 13 *33 1040 895 79 8 1.5 138 550 550 14
*34 1090 945 79 8 1.5 158 550 550 15 *35 1040 895 79 8 1.5 138 550
550 16 *36 1050 905 79 8 1.5 162 500 500 17 *37 1040 895 79 8 1.5
138 550 550 18 *38 1040 895 79 8 1.5 140 545 545 19 *39 1040 895 79
8 1.5 138 550 550 20 *40 1040 895 79 8 1.5 138 550 550 21 *41 1060
915 61 15 1.5 174 480 480 22 *42 1040 895 79 8 1.5 138 550 550 23
*43 1040 895 79 8 1.5 138 550 550 24 44 1040 895 79 8 1.5 138 550
550 25 44 1040 895 79 8 1.5 198 400 400 26 44 770 625 79 8 1.5 30
550 550 27 44 790 645 79 8 1.5 38 550 550 28 44 770 625 79 8 1.5 30
550 550 The mark "*" indicates that the value fell out of the range
defined in the present invention.
On each of cross sections of the resultant hot-rolled steel sheets
parallel to the rolling direction, a steel micro-structure was
observed to measure an area fraction of structures satisfying:
major grain diameter/minor grain diameter being 5.0 or more at
positions of 0.25 t (t: sheet thickness) and 0.50 t (t: sheet
thickness), and an average value of the area fractions was
determined. Next, on each of cross sections of the resultant
hot-rolled steel sheets parallel to the sheet thickness direction,
grain boundaries were exposed by nitric-acid electrolytic etching,
a line parallel to the sheet thickness direction was observed
within a range of 0.25 t to 0.75 t (t: sheet thickness), and a
number of grain boundaries crossing the line was measured to
determine the "average minor grain diameter." In addition, from
each of the resultant hot-rolled steel sheets, a Charpy impact test
specimen was taken and subjected to the Charpy impact test at
25.degree. C. Results of the above are shown in Table 4.
TABLE-US-00004 TABLE 4 STEEL MICRO-STRUCTURE STRUCTURES SATISFYING
MAJOR AXIS/ EVALUATION MINOR AXIS CHARPY IMPACT RUN Steel BEING 5.0
OR AVERAGE MINOR SURFACE VALUE AT 25.degree. C. NUMBER No. MORE
(area %) AXIS (.mu.m) QUALITY (J/cm.sup.2) INVENTIVE 1 1 95 80 GOOD
40 EXAMPLE 2 2 90 65 GOOD 130 3 3 98 70 GOOD 40 4 4 95 50 GOOD 80 5
5 90 48 GOOD 140 6 6 98 65 GOOD 40 7 7 97 80 GOOD 150 8 8 90 90
GOOD 40 9 9 96 95 GOOD 40 10 10 90 85 GOOD 150 11 11 98 70 GOOD 110
12 12 97 62 GOOD 50 13 13 70 80 GOOD 120 14 14 96 87 GOOD 150 15 15
95 86 GOOD 40 16 16 90 89 GOOD 90 17 17 98 92 GOOD 130 18 18 97 89
GOOD 40 19 19 90 90 GOOD 40 20 20 96 85 GOOD 110 COMPARATIVE 1 *21
85 105* GOOD 20 EXAMPLE 2 *22 80 110* GOOD 30 3 *23 75 120* GOOD 20
4 *24 65* 140* GOOD 25 5 *25 85 200* GOOD 30 6 *26 90 180* GOOD 25
7 *27 80 150* GOOD 10 8 *28 90 140* GOOD 15 9 *29 80 120* GOOD 20
10 *30 90 156* GOOD 25 11 *31 90 160* GOOD 30 12 *32 90 170* GOOD
25 13 *33 95 180* GOOD 20 14 *34 95 130* GOOD 25 15 *35 95 158*
GOOD 30 16 *36 95 180* GOOD 25 17 *37 97 170* GOOD 10 18 *38 95
120* GOOD 25 19 *39 95 180* GOOD 5 20 *40 92 170* GOOD 25 21 *41 95
168* GOOD 20 22 *42 98 210* GOOD 10 23 *43 90 180* GOOD 21 24 44 90
175* GOOD 10 25 44 98 180* GOOD 10 26 44 95 186* HOT ROLLING FLAW
12 27 44 95 171* HOT ROLLING FLAW 21 28 44 92 175* HOT ROLLING FLAW
13 The mark "*" indicates that the value fell out of the range
defined in the present invention.
As illustrated in Table 4, in Inventive Examples of the present
invention 1 to 20, their steel sheets all had good surface
qualities, and their Charpy impact values at 25.degree. C. were 40
J/cm.sup.2 or more. In contrast, in Comparative Examples 1 to 25,
at least one of their chemical compositions or steel
micro-structures fell out of corresponding ranges defined in the
present invention, and their toughnesses deteriorated. In addition,
in Comparative Examples 26 to 28, their temperatures of the rough
rolling were excessively low, which did not bring about the
recrystallization and coarsened grains, causing hot rolling flaws,
and their toughnesses also deteriorated.
INDUSTRIAL APPLICABILITY
According to the present invention, it is possible to provide
efficiently a ferritic stainless steel sheet excellent in
toughness. The ferritic stainless steel sheet is particularly
suitable to an automobile exhaust flange member.
* * * * *